This invention relates to electronic amplifier circuits having a regulatable gain, and in particular to a gain regulation circuit for use in a telephone instrument and whereby the gain of the instrument speech paths may be adjusted e.g. for matching the receiver gain to the line loop impedance.
An electronic telephone instrument includes an amplifying speech path whereby speech signals are relayed from the instrument microphone to the line and from the line to the instrument receiver. It will be appreciated that for optimum performance of the instrument the receiver and transmitter speech paths must each include some means of gain regulation to prevent overloading and to provide matching of the receiver gain to the line impedance. The line or loop impedance is in general proportional to the length of the subscriber loop between the instrument and the exchange and can thus have any value within a clearly defined range. Because it is necessary to cater for a wide variation of loop length, it is essential that the receiver circuit of the subscriber's instrument incorporates provision for line length compensation. To compensate loop length variations many telephone administrations require the provision of receiver circuits that can be set to a gain figure anywhere between upper and lower preset limits.
Furthermore, in order to prevent overloading and consequent saturation of the transmitter output state to the line it is necessary to provide some form of gain control whereby a speech signal waveform may be limited in amplitude. This technique commonly known as soft clipping is described in my co-pending application Ser. No. 491,834, filed May 5, 1983.
A particularly useful variable gain element is the transistor long tailed pair the gain of which can be determined by controlling the common emitter current of the pair. The use of a single long tailed pair is, however, somewhat restricted by its significantly non linear signal/gain characteristic which gives rise to distortion for all but very low signal levels. In some applications this distortion may be outside the limits specified by the telephone administration.